The invention relates to a fill level measuring system for the liquid content of a tank (tank indicator), with a fill level sensor for generating a signal proportional to the height of a point of the liquid level, an additional volume flow sensor for generating a signal proportional to the inflow or outflow, and an indicating element.
In the numerous fields of technology there is a need to determine the liquid content of a tank. While it is possible to determine the weight in the case of smaller tanks, a level measurement is generally carried out for larger tanks. For this purpose, fill level sensors are used that generate a signal proportional to the height of a point of the liquid level. Such fill level sensors for a level measurement are typically lever float sensors in which a float is connected via a lever with a potentiometer, or rather capacitors that are designed in tubular form, for example, and a allow a capacitive fill level measurement in that the liquid to be measured acts as a dielectric.
In numerous applications, however, the liquid surface of the liquid is exposed in the tank to high-frequency or low-frequency excitation, in such a way that determining the liquid level with the precision to be required for various applications becomes impossible with fill level sensors. This applies in particular to the determining of fuel supply quantities via a level measurement of the fuel in the tank of a vehicle. This includes combustion-engine-propelled land vehicles as well airplanes or ships. The level of the fuel liquid in supply tanks in all vehicles is subject to constant fluctuations due to position and acceleration changes. An indicator aligned undamped on the given level thus fluctuates considerably and leads to problems in particular where overestimating the remaining fuel quantity may lead to dangerous situations, in the case of motor-driven airplanes.
Since the fuel tank in most cases is solidly built into the respective vehicle, into an airplane for example, the accelerations and position changes of the respective vehicle, in particular an airplane, and thus also those of the tank have a direct effect as input values on the movement of the fluid. Specifically in the case of airplanes, high-frequency disturbances of the liquid surface can be observed that are attributable to air turbulence, as well as low-frequency disturbances that are brought about, for example, by lasting position changes of the vehicle, in climb or the like, for example.
FIG. 7 shows as an example the fill level sensor voltage of a typical float lever sensor of a small airplane during a horizontal flight of roughly 15 seconds. The illustration clearly shows high-frequency disturbances with a frequency of approx. 1 Hz that are excited by air turbulence. The fill level sensor voltage curve shows amplitudes of up to .+-.125 mV, which corresponds to a fluctuation of .+-.1.3 1 for the model of airplane examined. In flights in turbulent weather situations, fluctuations of .+-.10 1 to .+-.15 1 were observed in the fuel gauge, for an approx. 75 1 total fuel volume of the small airplane observed.
FIG. 8 shows typical steady deviations that result from very lengthy, extremely low-frequency flight position changes (e.g. steady climb, descent or slow flights). The formation of such deviations depends first and foremost on the tank's geometry, the mounting site of the fill level sensor, the momentary fuel volume and the tank's angle of inclination. FIG. 8 shows the absolute deviation of the scanning height depending on the airplane's longitudinal angle of orientation and the tank content. For example, one can see that with a 16.degree. longitudinal angle of orientation of the airplane and an approx. 39 1 momentary tank volume for this tank, a 30 mm error of the float excursion occurs, tantamount to an error volume of approx. 11 1 for the geometry of this specific tank. For this reason, the fuel quantity is overestimated by approx. 27%. This can lead to a significant miscalculation by the pilot with respect to the airplane's range available to him. Similar systematic errors result in the case of vehicles on an upward run or downward run, for example.
A fill level indicator is known from DE 38 25 630 C2, in which the conventional fill level measuring sensor, designated there as a static transmitter, is supplemented by a consumption measuring system that determines the momentary fuel consumption based on injection signals of a fuel injection system. In this connection, the tank indicator is essentially controlled by the integrated volume flow which is subtracted from a supply quantity of the fuel determined at specific points in time by the static transmitter. Instead of using the injection signals to determine the fuel consumption, in general a flowmeter is also proposed, and hence a volume flow sensor in the wording of the patent application.
A considerable disadvantage in the fill level measuring system proposed in the form of 38 25 630 C2 according to the state of the art is the fact that the volume flow sensor must be of high quality in order to deliver utilizable results. Since linearity deviations of the volume flow sensor are integrated, and thus lead to very high absolute deviations of the tank indication as time goes on, only volume flow sensors with a linearity error in the low single-digit percent range are acceptable. Such volume flow sensors are very costly and thus are out of the question for use in vehicles and small airplanes, motor boats, etc.
The invention is thus based on the technical problem of improving a fill level measuring system of the generic type in such a way that comparably inexpensive components with comparably high equipment-specific deviations can be used as fill level sensors and volume flow sensor, and in spite of this a precise tank indication is made possible. In this connection, high-frequency disturbances should continue to be eliminated in such a way that a steady tank indication is ensured. Furthermore, low-frequency disturbances, such as lengthy position changes of the airplane with lesser angular frequency (climb/descent) are considerably attenuated with respect to their effects on the measuring result, without any lag error being generated due to the considerable attenuation.
The solution of the technical problem is characterized in that in a generic fill level measuring system, the signal of the fill level sensor is directed through a low-pass filter and the signal of the volume flow sensor is directed through a high-pass filter, and that both signals are summed up before the indicating instrument. In this connection, a preferred and advantageous dimensioning is for the cut-off frequency (time constant) of the low-pass filter to be essentially identical to the cut-off frequency (time constant) of the high-pass filter. With such a dimensioning it can be achieved that low-frequency disturbances are eliminated by designing the low-pass filter accordingly, in connection with which, lag errors occurring with the extreme dimensioning of the low-pass filter are compensated by the signal portion supplied to the high-pass filter. The time constants resulting during typical applications are so great that conventional analog components, in particular capacitors, attain sizes and weights that are unacceptable. According to a preferred form of construction it is thus provided for that the filters are digitally constructed. In this connection, it is particularly preferred that when using the invention's fill level measuring system for indicator systems in airplanes, the time constants of the filters are able to be optimized with respect to a typical flight conduct. Thus, for example, a training airplane that often circles and flies start/landing exercises can be optimized with respect to its fill level measuring system differently than an airplane intended for lengthy cruising.
There are considerable delays after starting the fill level measuring system according to the invention due to the considerable damping of the low-pass. For this reason, it may be provided for that the low-pass filter is able to be bridged briefly starting compensation, in order to quickly provide a first reading to the pilot or other user of a vehicle.
The planned supplementary filtering (low-pass filtering for the signal of the fill level sensor/high-pass filtering for the signal of the volume flow sensor) can also be achieved indirectly in that the signal of the fill level sensor is used as a support value for a gauging filter that has an integrator adjacent to the input of which is the difference from the signal generated by the volume flow sensor and a returned correction signal, in connection with which the latter signal is proportional to the difference between the momentary fill level sensor signal as a support value and the output of the integrator as an estimated value.
Such a gauging filter also leads to a low-pass filtering of the fill level sensor's signal with low cut-off frequency (high time constant), in connection with which a systematically resulting lag error is compensated by a supporting of the filter with the volume flow sensor signal. In this connection, the supporting causes a high-pass-like filtering of the volume flow sensor signal.
It is particularly advantageous to use as fill level sensor a basically known sensor that has a potentiometer and a float attached to a lever.
In particular the volume flow sensor may be an inexpensive turbine wheel meter or a hot-wire anemometer.
Further preferred forms of construction of the invention can be inferred from the subclaims.